This invention relates to a control circuit for controlling the self-cleaning function of a humidity sensor, and its object is to switch the characteristics of a control circuit for a ceramic humidity sensor having a self-cleaning function through changes in a resistance value from the self-cleaning mode to the humidity detecting mode or vice versa so as to thereby ensure an accurate control of the temperature of its humidity responsive region in its self-cleaning mode and an improved humidity detecting performance in its humidity detecting mode.
With advances in the art of sensors and microcomputers, an automated cooking appliance has been put into practical use in which a humidity sensor is provided for sensing variations in the relative humidity in a heating chamber which are caused by the steam generated from the cooking food so as to thereby monitor the progress of cooking and then automatically detect the completion of the cooking or enabling the switching of an output level.
The humidity sensor, because of its very nature as a humidity detecting means, is constantly exposed to the smoke, steam, oil and so forth generated from the cooking food, with the consequence that the humidity responsive region of the sensor becomes fouled and unable to sense the humidity accurately after a long time of use. For a sensor whose humidity responsive region is made of ceramic material, that is, a so-called ceramic humidity sensor, a self-heating heater is installed on its humidity responsive surface and periodically energized to burn out such dirt and to restore its humidity responsive characteristics that is, a refresh operation of the sensor. A way to control such a heating temperature depends upon the thermistor property of the sensor.